drm/i915: Add helpers to reference/unreference a DPLL for a CRTC

Add helpers to reference/unreference a shared DPLL tracking the use of
it by a given CRTC.

This prepares for the next patch, which unreferences a DPLL during CRTC
HW-readout/sanitization.

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230510103131.1618266-3-imre.deak@intel.com